Total organic carbon (toc) analyzer

ABSTRACT

The invention disclosed is a total organic carbon (TOC) analyzer comprised of an electrochemical cell comprising a diamond-film electrode ( 2 ) doped with boron or other conductivity including material. The diamond-film electrode is the working electrode and carries out the oxidation of TOC to produce carbon dioxide. The apparatus further comprises sensors for detecting the carbon dioxide produced. Such sensors include but are not limited to a tunable diode laser ( 1 ) and/or ion-selective electrode ( 5 ). The invention also discloses a method for measuring TOC in an aqueous solution using a total organic carbon analyzer.

Government Support

This invention was made with government support under NAG5-11806 awardedby the National Aeronautics and Space Administration. The government hascertain rights in the invention.

BACKGROUND OF THE INVENTION

The measurement of total organic carbon (TOC) is frequently performed inenvironmental, clinical, and industrial settings. Current techniquesrequire hazardous reagents, such as strong acid and oxidizing agents,ultraviolet light, or high temperature ovens, to carry out the oxidationreactions. The development of a safe and cost-effective electrochemicaldevice capable of oxidizing organic carbon and determining TOC inaqueous solution would represent a significant advance in the art.

The concept of electrochemically oxidizing organics was demonstrated inthe early 1990's as a technology for incineration of toxic organicindustrial wastes. Unfortunately, this technology has thus far proveninefficient and of limited use. Over the years, a variety of workingelectrodes for electrochemically oxidizing organic carbon have beendeveloped. The properties of a working electrode in an electrochemicalcell is critically important since the working electrode is directlyinvolved in the oxidation of the organic molecule. The most commonworking electrode material has typically been carbon-based or made frommetals such as platinum, silver, gold, mercury, or nickel. Suchelectrodes, however, poorly oxidize because of their limited anodicrange. These electrodes eventually themselves become oxidized, andtherefore are inefficient for any practical use. To overcome theselimitations, recent attention has focused on the potential use ofdiamond-film electrodes. Such electrodes are composed of a substratematerial, such as silicon or titanium, coated with diamond. Suchelectrodes are made conductive by doping the diamond film with aconductivity inducing material which promotes p-type semiconductivity toalmost metallic levels (e.g., boron).

The unique properties of highly boron-doped diamond (BDD) films include:i) low and stable voltammetric and amperometric background currents, ii)wide working potential window in electrolyte solutions, iii) reversibleto quasi-reversible electron transfer kinetics for redox species, iv)morphological and microstructural stability at extreme anodic andcathodic potentials, v) low adsorption of polar molecules, and vi)long-term response stability. Recently it has been reported that BDDfilms have been coated on several substrates and used to replace earlierelectrodes (e.g., gold or platinum) for substrates for electrochemicaloxidation of organic wastes. In these uses, the BDD-film electrodes havebeen reported to be highly robust, capable of withstanding high anodicpotentials, and resistant to self-oxidation. An example is the use of aBDD-film electrode for the electrochemical oxidation of phenol. Cyclicvoltammetry showed that phenol, one of the most difficult organicmolecules to oxidize electrochemically, was oxidized by a BDD-filmelectrode with no visible oxidation of the electrode itself, even aftermultiple cycles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of the total organic carbonanalyzer apparatus.

SUMMARY OF THE INVENTION

The present invention relates to a total organic carbon analyzerapparatus for measuring total organic carbon (TOC) in an aqueoussolution. The apparatus comprises an electrochemical cell having adiamond-film electrode, a reference electrode, and a counter electrode.The diamond-film electrode, where the diamond is doped with boron orother suitable atoms which will raise the conductive band of thediamond, is the working electrode and carries out the oxidation of theorganic material to produce carbon dioxide. The apparatus furthercomprises a sensor or sensors for detecting the carbon dioxide producedby the diamond-film electrode. A suitable sensor is one that can detectcarbon dioxide at the levels generated by the BDD electrode. Examples ofsensors exhibiting this degree of sensitivity include a tunable diodelaser spectrometer (TDL) and an ion-selective electrode (ISE).

The instant invention also relates to a method for measuring TOC in anaqueous solution using a total organic carbon analyzer apparatus havinga diamond-film electrode. The method provides for immersion of thediamond-film electrode of the electrochemical cell into an aqueoussolution to be analyzed for TOC. A positive potential, in the range ofabout 2-2.5 volts, is applied to the diamond-film electrode to oxidizethe organics in the solution and produce carbon dioxide. As previouslymentioned, the amount of carbon dioxide produced is detected andmeasured using a carbon dioxide sensor. The amount of carbon dioxidemeasured is proportional to the amount of carbon oxidized and is used tocalculate the amount of total organics in the solution.

DETAILED DESCRIPTION OF THE INVENTION

Accurate detection and measurement of organics in an aqueous solutionhas historically required the use of costly methods involving largeequipment and the use of hazardous reagents. The use of electrochemicaloxidation of carbon with detection of the resultant carbon dioxide gas,promises a versatile, easy to use, and cost-effective alternative foraccurately determining TOC levels in a solution. While a variety ofelectrodes made of different materials are readily available, they arelimited in their capacity to effectively oxidize organics.

The present invention relates to the use of an electrochemical cell forcarrying out the efficient oxidation of organic carbon to water andcarbon dioxide. The working electrode is composed of a substratematerial, such as silicon, niobium, or titanium, coated with diamond.The electrode is made conductive by doping the diamond film with boron.The carbon dioxide produced at the electrode surface is detected usingan appropriate carbon dioxide sensor.

One aspect of the present invention relates to an apparatus formeasuring total organic carbon in an aqueous solution. Such an apparatusincludes two principle components contained within a suitable sealedhousing to create a closed system so that carbon dioxide generatedthrough the oxidation process can not escape from the system prior todetection. The selection of housing and sealing elements are matters ofdesign choice. The first principle component, an electrochemical cell,is comprised of a working electrode, a reference electrode, and acounter electrode. The working electrode is a diamond-film electrodethat is suitable for oxidation of organics in an aqueous sample toproduce carbon dioxide. As previously mentioned, the diamond-film isdoped with a conductivity inducing material, such as boron.

The second principle component, a carbon dioxide sensor, is used todetect the carbon dioxide produced when the electrochemical celloxidizes the organics in the sample solution. Carbon dioxide sensorshaving an appropriate degree of sensitivity for use in connection withthe present invention are available in either a gas-phase oraqueous-phase format. An example of a suitable gas phase carbon dioxidesensor is a tunable diode laser (TDL) spectrometer, and an example of asuitable aqueous-phase carbon dioxide sensor is an ion-selectiveelectrode (ISE). Both the TDL and ISE sensors are known in the art. Thedevice of the present invention may include one or more carbon dioxidesensors. In preferred embodiments, where only one sensor is provided,that one sensor is a gas-phase carbon dioxide sensor. The gas phasesensor is used to detect carbon dioxide gas bubbled from the aqueoussolution containing the organic being oxidized. To quantitate levels oforganic in the aqueous solution using a single gas phase sensor, it isnecessary to convert substantially all of the organic material in thesample to carbon dioxide and water, and to allow sufficient time fordissolved carbon dioxide to bubble from solution. Alternatively, acombination of a gas-phase sensor and an aqueous-phase sensor may beused to quantitate organics in solution. When a combination of sensorsis employed, it is not necessary to allow all dissolved carbon dioxideto bubble into the head space of the closed system (see FIG. 1, forexample) wherein it can be measured using a gas-phase sensor. Rather, aliquid-phase sensor (e.g., an ISE) is useful for measuring levels ofdissolved carbon dioxide in the aqueous-phase, and the gas-phase sensor(e.g., a TDL) measures the gas phase carbon dioxide in the head space.Assuming that the conversion from organic to carbon dioxide and water issubstantially complete, the sum of the gas-phase and aqueous-phasecarbon dioxide levels can be used to calculate concentrations of theorganic originally present in the aqueous sample.

Such an apparatus, comprising an electrochemical cell having a workingelectrode made of boron-doped diamond and a TDL carbon dioxidespectrometer or ISE, may be manufactured to be compact and of relativelysmall size as compared to existing devices intended for similarapplication. Given the relatively compact formats achievable based onpresent disclosure, the device of the present invention is particularlywell-suited for portable field use.

Another aspect of the present invention relates to a method formeasuring total organic carbon in an aqueous solution. First, thediamond-film electrode is brought into contact with a volume of a sampleto be tested for its level of total organics. The sample is an aqueoussolution, such as water from a municipality, or a solution containing adissolved specimen.

The electrode may be immersed into the sample solution or a flow-cellmay be used, where the sample is circulated through the cell and incontact with the diamond-film electrode. To achieve optimal oxidation oforganics in the sample, a positive potential in the range of about 2-2.5volts is applied to the diamond-film electrode. In preferredembodiments, the positive potential is provided by an external battery.This current will cause oxidation of organics to occur, producing carbondioxide at the surface of the diamond-film electrode. The carbon dioxidemay bubble into a collector chamber (e.g., the headspace above theaqueous solution in FIG. 1) in communication with the carbon dioxidesensor where it is measured spectroscopically and recorded in absorptionunits (AU). The carbon dioxide may also be dissolved in the aqueoussolution where it can be measured by an ISE selective for carbon dioxideand recorded as an electric potential in volts (V).

The TDL may be a small laser diode that produces a very narrow andspecific wavelength of light tuned to the harmonic frequency of thecarbon dioxide gas molecule in the near infrared band. To specificallymeasure carbon dioxide with a TDL, the TDL is preferably tuned to theharmonic frequency of carbon dioxide molecules in the near-infraredband. After being adjusted to the carbon dioxide frequency, the tunablelaser diode may be tuned to different wavelengths on either side of thecarbon dioxide wavelength. The light energy being absorbed by the carbondioxide is then compared to calibrated values at the surroundingfrequencies to obtain a precise quantitative measurement of the amountof carbon dioxide produced. The amount of carbon dioxide measured willbe directly proportional to the amount of organics that were present inthe aqueous sample. Using aqueous samples containing known quantities ofdissolved organics, a standard curve may be generated to allow for thedetermination of precise concentration of organics within the testsolution.

An Ion Selective Electrode (ISE) is a membrane electrode that respondsselectively to certain ions in the presence of other ions. The ISE isparticularly useful in the present invention for the detection ofdissolved carbon dioxide generated by the breakdown of organic compoundsat the working electrode. Such dissolved carbon dioxide may be measuredby the ISE prior to its buildup in the headspace above the samplesolution, where the TDL is useful. To specifically measure carbondioxide with an ISE, the membrane of the ISE is selected such thatcarbon dioxide molecules selectively cross the membrane—such ISEsselective for carbon dioxide are well known in the art. The ISE isbrought into contact with the sample solution and the potential at theISE is then compared to that of the reference electrode. The magnitudeof the potential at the ISE vs. the reference electrode is directlyproportional to the concentration of carbon dioxide dissolved in theaqueous sample. The amount of carbon dioxide measured will be directlyproportional to the amount of organics that were present in the aqueoussample. Using aqueous samples containing known quantities of dissolvedorganics, a standard curve may be generated to allow for thedetermination of precise concentration of organics within the testsolution.

FIG. 1 is a diagrammatic representation of an embodiment of the presentinvention. The apparatus comprises an electrochemical cell having adiamond-film electrode 2, a reference electrode 4, and a counterelectrode 3. The diamond-film electrode 2 is the working electrode andcarries out the oxidation of the organic material to produce carbondioxide. The apparatus further comprises a carbon dioxide sensor fordetecting the carbon dioxide produced by the diamond-film electrode.Examples of suitable sensors include a tunable diode laser spectrometer(TDL) 1 and an ion-selective electrode (ISE) 5.

1. An apparatus for measuring total organic carbon in an aqueoussolution, comprising: a) an electrochemical cell comprising: i) adiamond-film electrode; ii) a reference electrode; and iii) a counterelectrode; and b) one or more carbon dioxide sensors, including at leastone gas-phase sensor.
 2. The apparatus of claim 1 wherein thediamond-film electrode is doped with a conductivity inducing material.3. The apparatus of claim 1 wherein the gas-phase sensor is a tunablediode laser spectrometer.
 4. The apparatus of claim 1 further comprisingan aqueous-phase carbon dioxide sensor.
 5. The apparatus of claim 4wherein the aqueous-phase sensor is an ion-selective electrode.
 6. Theapparatus of claim 2 wherein the conductivity inducing material isboron.
 7. A method for measuring total organic carbon in an aqueoussolution, the method comprising: a) providing an electrochemical cellcomprising of: i) a diamond-film electrode; ii) a reference electrode;and iii) a counter electrode; b) immersing the electrochemical cell ofstep a) into the aqueous solution; c) applying a positive potential tothe diamond-film electrode, the positive potential being sufficient tooxidize organics in the solution, thereby producing water and carbondioxide; d) measuring the amount of carbon dioxide produced in step c)using one or more carbon dioxide sensors, including at least onegas-phase sensor; and e) determining the amount of total organic carbonin the solution, the amount of total organic carbon being proportionalto the amount of carbon dioxide measured in step d).
 8. The method ofclaim 5 wherein the gas-phase sensor is a tunable diode laserspectrometer.
 9. The method of claim 5 wherein the diamond-filmelectrode is doped with a conductivity inducing material.
 10. The methodof claim 9 wherein the conductivity inducing material is boron.
 11. Themethod of claim 5 wherein the positive potential is in the range ofabout 2-2.5 volts.
 12. A method for measuring total organic carbon in anaqueous solution, the method comprising: a) providing an electrochemicalcell comprising of: i) a diamond-film electrode; ii) a referenceelectrode; and iii) a counter electrode; b) immersing theelectrochemical cell of step a) into the aqueous solution; c) applying apositive potential to the diamond-film electrode, the positive potentialbeing sufficient to oxidize organics in the solution, thereby producingwater and carbon dioxide; d) measuring the amount of carbon dioxideproduced in step c) using one or more carbon dioxide sensors, includingat least one gas-phase sensor, and at least one aqueous-phase sensor;and e) determining the amount of total organic carbon in the solution,the amount of total organic carbon being proportional to the amount ofcarbon dioxide measured in step d).
 13. The method of claim 12 whereinthe aqueous-phase sensor is an ion-selective electrode.